1. Field of the Disclosure
The present disclosure generally relates to spectroscopy systems and, more particularly, to a single detector based spectroscopy system employing dual illumination sources along with a configurable holographic spectrum analyzer for customizable simultaneous spectral displays.
2. Brief Description of Related Art
Modem spectroscopy systems rely on various optical properties of samples under investigation to elicit information about a sample's chemical and physical structure (e.g., chemical, molecular, or elemental composition), characteristics, and properties. A sample's spectrum may provide one or more of such information depending on the spectroscopy method employed for the investigation. Some examples of spectroscopy methods include Raman spectroscopy, infrared spectroscopy, fluorescence spectroscopy, mass spectrometry, x-ray diffractometry, etc. Each spectroscopy method may have a different level of specificity or discriminatory power. For example, a Raman spectrum generally has higher discriminatory power than a fluorescence spectrum and, hence, Raman spectroscopy is generally considered more specific for the identification of an unknown material. However, Raman scattered signals are typically far weaker than fluorescence emissions and, hence, Raman spectroscopy systems may require more sensitive signal collection optics and more selective data processing techniques to collect and process weaker Raman signals. Thus, pros and cons of various spectroscopy systems (e.g., cost, speed, accuracy, specificity, sensitivity, etc.) should be considered prior to selecting a spectroscopy approach for the application at hand.
FIG. 1 illustrates a prior art fiber array spectral translator (FAST) based spectroscopy system 10 that may be used for Raman spectroscopy. In the system 10, an object or sample 12 is shown to receive illumination from an illumination or excitation source 16 (e.g., a monochromatic illumination or laser illumination). Radiation reflected, emitted, or scattered from various illuminated regions in the sample 12 may be collected by collection unit 20 (e.g., a microscope objective) before the collected signal is fed to a two-dimensional (2D) array 22 of optical fibers as indicated by exemplary arrows 18 and 19 depicting optical signal propagation path from the sample 12 to the 2D fiber array 22. Three exemplary regions in the sample 12 are indicated by reference numerals 14A (cross-hatched circle), 14B (filled circle), and 14C (a circular ring). In the FAST system 10, one end of the fiber bundle (i.e., the end receiving optical signals from the collection unit 20) is shown formed in the shape of a 2D array of optical fibers 22, whereas the other end of the fiber bundle is shown arranged in a one-dimensional (1D) curvilinear array 24. Thus, the FAST system 10 may be considered to transform a 2D field of view (FOV) into a ID arrangement at the entrance of a spectrometer slit 27 as shown in FIG. 1.
From the exemplary layout in FIG. 1, it is seen that each fiber in the 2D fiber bundle 22 may image a certain portion of the sample-as indicated by linking of three exemplary sample regions 14A-14C with corresponding fibers in the 2D fiber bundle-here, fibers numbered “21,”“18,” and “7,” respectively in the exemplary set of thirty fibers (numbered from “1” through “30” in FIG. 1) in the 2D fiber bundle 22. The optical data set (i.e., photons reflected, emitted, or scattered from the illuminated sample 12) collected by all the thirty (30) fibers at the 2D end 22 is fed through the 1D curvilinear end 24 into the slit 27 of the spectrograph 26. The spectrograph 26 may be a holographic grating-based spectrometer, which may be coupled to an optical detector 28 (e.g., a charge coupled device or CCD) to store fiber-specific spectral information in the corresponding CCD rows as illustrated by three exemplary highlighted bars 14A-14C in a layout 30 of a CCD output array having an X-Y resolution of 1024×1024 pixels. In the layout 30, the reference numerals 14A-14C are used to indicate how correspondence is maintained among the similarly-numbered regions of interest in the sample 12, the corresponding fibers receiving optical data from these regions of interest (i.e., the fibers numbered “21,” “18,” and “7,” respectively), and the CCD rows storing fiber-specific spectral information. Thus, a one-to-one correspondence between a CCD row and a sample region of interest may be achieved for spatially-resolved spectral data collection. Thus, as shown for example in FIG. 1, spectral information from the region 14A in the sample 12 may be collected by the fiber #21 (in the exemplary set of 30 fibers shown in FIG. 1) and may be stored as corresponding electrical charge in one of the rows in the CCD. Similarly, spectral content from other regions on the sample may also be stored in corresponding CCD rows to obtain a 2D image of the entire illuminated FOV (field of view) of the sample.
It is observed here that a third dimension for the 2D CCD layout 30 may be defined from the spectral information of the entire illuminated FOV of the sample 12. In the exemplary illustration in FIG. 1, each row in the CCD layout 30 is numbered from 1 through 30 to correspond with the total number of fibers in the FAST fiber bundle. Thus, it is understood that each CCD row represents a fiber-specific output from a fiber associated with that row, and that each fiber-specific output may contain a plurality of wavelengths that may be represented as various “columns” (not shown in the layout 30 in FIG. 1) in the CCD output 30. Hence, each CCD column can be redefined to a wavelength-specific 2D spatial image corresponding to the fiber bundle architecture and CCD storage mechanism. A plurality of such wavelength-specific 2D spatial images 32-1 through 32-n are illustrated in FIG. 1 as forming a hyperspectral image cube 34 formed from the spectral data stored in various CCD “columns” (along the X-axis), wherein, as noted before, each CCD column represents a wavelength-specific spatial (image) information collected by the FAST fibers and stored along the Y-axis. The third dimension (the Z-axis) may represent the individual wavelengths and can be defined by the spectral information of the CCD rows. This spectral information represents the material characteristic information of the illuminated FOV of the sample 12 as is known in the art. In the exemplary hyperspectral image cube 34 in FIG. 1, the mapping of spectral data from a sample region of interest across all wavelengths (along the Z-axis) is illustrated by dotted lines linking a spatial location of a region of interest (e.g., the region 14A) through all the spatial image frames 32-1 through 32-n. 
It is noted here that all the figures herein are for illustrative purpose only; the figures are not drawn to scale and nor do they depict complete hardware or spectral details. Furthermore, the exemplary Raman spectra illustrated in various figures are depicted after appropriate removal of fluorescence and after baseline adjustments are carried out.
FIG. 2 depicts an exemplary Raman spectrum 40 collected from a sample 12 by one of the fibers in the FAST system 10 of FIG. 1. As discussed before, this spectrum may represent charge collected at one of the CCD rows corresponding to the fiber at issue. The entire CCD may thus contain spectral data from many such fibers—each CCD row may contain information to represent a similar Raman spectrum. Thus, in one embodiment, the spectrum 40 may represent an average of all Raman spectra collected by all FAST fibers in the system 10. As noted before, the fibers may collect light reflected, emitted, or scattered from the sample 12. Therefore, the actual spectral data content in the CCD rows may reflect fluorescence data as well as Raman data. The spectrum 40 in FIG. 2 is obtained after appropriate removal of the fluorescence portion and after the baseline adjustments are carried out. These procedures are not illustrated or discussed herein for the sake of brevity.
It is seen from FIG. 2 that a “typical” Raman spectrum 40 may include a fingerprint region 42, a C-H (carbon-hydrogen) stretch region 44, and a substantially less significant intervening region 46. It may be desirable to focus on or explore the fingerprint region 42 (which represents more information about chemical and physical characteristic of a sample under investigation), and sometimes the C-H stretch region 44 (which may provide information about carbon-hydrogen bonds in the sample material), in more detail in many spectroscopy applications. It may be further desirable to be able to have the fingerprint region 42 and the C-H stretch region 44 displayed simultaneously on a single display monitor or screen. For additional spectroscopic analysis, it may be also desirable to have simultaneous display of two user-selected spectral regions of interest, which may not necessarily include the fingerprint region 42 or the C-H stretch region 44.
In case of further spectroscopic exploration of chemical or physical properties of the sample 12, it may be desirable to devise a system containing multiple illumination sources so that the sample 12 can be illuminated with different laser excitation wavelengths and resulting multiple sample spectra can be comparatively observed for a more fruitful analysis of sample spectral responses (and, hence, sample properties) under different excitations. In such an event, it may be further desirable to be able to focus on selected spectral regions of interest from different such spectra and have a simultaneous display of such selected spectral regions of interest so as to enable a user to perform a better comparative analysis when spectra from different illumination sources are juxtaposed with each other on a single electronic display.